Possibly buying a motor yacht with damaged prop shaft

In fact you're going to have to specify to the surveyor that there's possible damage in this area and he's to provide full in-depth report with comparative measurements between port and starboard sides.
This will be an adder to his survey fee so don't be surprised if an additional $1000 or so . But without definitive measurements you'll get a a " looks okay" report which tells you very little.

One of our customers motor mounts was damaged by a loose dock line.

The next question that needs to be asked; was the boat operated after the damage, then look carefully at the clutch flange and output shaft bearings. This requires lots more survey time.

I wouldn’t spend a dime on survey unless the agreed price was $10/15k below comparables. Too many things could go wrong

I ve never got my own rode caught by a wheel but I ve had lines caught around a shaft and stall a motor. Didnt bend the shaft even though one time the line was tied to a piling. **** neighbor never hooked his lines right... I know Silvertons are built light but I wonder if maybe they re not too weak

So the shaft got damaged because of a line that allegedly didn’t stall the engine? Seriously? Either the seller is lying or Silvertons are built even lighter than I thought.

As I said earlier if it is priced 10/15k below value... go ahead and gamble. Otherwise get a real boat. Pardon me... a real motoryacht.

How does one foul an anchor line around a shaft without the engine running and shaft rotating?
For the survey to assess for damage you will have to haul the boat. Is the seller paying for this until and if you buy the boat? If you haul, find damage and walk away, are you out the cost of the haul? Has the seller even hauled the boat since this incident?

Well then do a sea trial before survey. If you don’t have any vibration then go ahead and survey.

if you get vibration then get the seller so solve it

Something to ponder;
It was fine before a line got tangled.
Now, after the tangle, it is not fine.
Tangles do not make engines out of align.

Something is damaged.

Have your mechanical surveyor watching the shaft and motor while underway to the lift.
He can advise you weather to haul or not.
If it is bad, you need to determine to continue spending money for the haul out inspection and further surveys.

Under value means nothing. Hate that term.
What is it worth to you and your insurance company is the money value.

Purchasing a broken boat with issues and other surprises, offers no savings in value.

Let me repeat what said earlier.

DO A SEA TRIAL!

that is pretty standard for any purchase anyway. Then you can decide whether you want spend money on surveys and haul out.